# Total number of non-decreasing numbers with n digits

A number is said to be non-decreasing when all digits (except the first place) is not smaller than its previous digit. For this algorithm, we have to find how many nondecreasing numbers are there in an N-digit number.

Let count(n, d) a function to count how many non-decreasing numbers are there of length n and ending with letter d, then we can write the relation like this.

$$count(n,d)=\displaystyle\sum\limits_{i=0}^d count(n-1,i)\total=\displaystyle\sum\limits_{d=0}^{n-1} count(n-1,d)$$

## Input and Output

Input:
Number of digits, say 3.
Output:
The possible non decreasing numbers. Here it is 220.
Non decreasing numbers are like 111, 112, 123, 789, 569 etc.

## Algorithm

countNumbers(n)

Input: The given value.

Output: Number of non-decreasing values from n digit numbers.

Begin
define count matrix of order (10 x n+1), and fill with 0
for i := 0 to 9, do
count[i, 1] := 1
done

for digit := 0 to 9, do
for len := 2 to n, do
for x := 0 to digit, do
count[digit, len] := count[digit, len] + count[x, len-1]
done
done
done

nonDecNum := 0
for i := 0 to 9, do
nonDecNum := nonDecNum + count[i, n]
done

return nonDecNum
End

## Example

#include<iostream>
using namespace std;

long long int countNumbers(int n) {
long long int count[n+1];    //to store total non decreasing number starting with i digit and length j

for(int i = 0; i<10; i++)
for(int j = 0; j<n+1; j++)
count[i][j] = 0;     //initially set all elements to 0

for (int i = 0; i < 10; i++)    //set non decreasing numbers of 1 digit
count[i] = 1;

for (int digit = 0; digit <= 9; digit++) {     //for all digits 0-9
for (int len = 2; len <= n; len++) {       //for those numbers (length 2 to n)
for (int x = 0; x <= digit; x++)
count[digit][len] += count[x][len-1];     //last digit x <= digit, add with number of len-1
}
}

long long int nonDecNum = 0;

for (int i = 0; i < 10; i++)    //total non decreasing numbers starting with 0-9
nonDecNum += count[i][n];
return nonDecNum;
}

int main() {
int n = 3;
cout << "Enter number of digits: "; cin >> n;
cout << "Total non decreasing numbers: " << countNumbers(n);
}

## Output

Enter number of digits: 3
Total non decreasing numbers: 220